Númenor aids Gil-galad - Sauron destroyed Eregion, but he still lacked that which he was seeking, the Three Elven Rings. Though Celebrimbor didn't tell him outright where they were, he knew Eregion had been an outpost of Lindon.

"Concerning the Three Rings Sauron could learn nothing from Celebrimbor; and he had him put to death. But he guessed the truth, that the three had been committed to Elvish guardians; and that must mean to Galadriel and Gil-galad...Now Sauron's immediate purpose was to take Lindon, where he believed he had most chance of seizing one, or more, of the Three Rings; and he called in therefore his scattered forces and marched west towards the land of Gil-galad, ravaging as he went."~Unfinished Tales (The History of Galadriel and Celeborn)

In the First Age, the Eldar and Edain had to join forces to fight Morgoth. Now they had to do so once again to fend off the threat of Sauron.

"As soon as Gil-galad began to fear that Sauron would come with open war into Eriador he sent messages to Númenor; and on the shores of Lindon the Númenóreans began to build up a force and supplies for war. In 1695, when Sauron invaded Eriador, Gil-galad called on Númenor for aid. Then Tar-Minastir the King sent out a great navy; but it was delayed and did not reach the coasts of Middle-earth until the year 1700. By that time Sauron had mastered all Eriador, save only besieged Imladris, and had reached the line of the River Lhȗn. He had summoned more forces which were approaching from the south-east, and were indeed in Edenwaith at the crossing of Tharbad, which was only lightly held. Gil-galad and the Númenóreans were holding the Lhȗn in desperate defence of the Grey Havens, when in the very nick of time the great armament of Tar-Minastir came in; and Sauron's host was heavily defeated and driven back."~Unfinished Tales (The History of Galadriel and Celeborn)